I think a lot of the variation in people's experiences with this is because of variation in the natural air humidity. If it's humid, your breath will cause a lot of condensation. In cold dry air, such as I usually get in the upper midwest, and combined with the newer water-resistant down, I think it is perfectly OK to do this at least to occasionally warm up your nose. As others have stated, I would be careful about doing it all night, or on a multi-day trip.
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